Electromagnetic noise is emitted when a motor is operating. One element of electromagnetic noise is harmonic noise. Harmonic noise is mainly caused by the motor's torque ripple. For example, when a permanent magnet direct current (PMDC) motor is operating, the position of the coils with respect to the stator and the current through the windings varies periodically, which results in a periodic torque ripple. FIG. 3 is a cross sectional view of a twelve slot rotor core of a conventional motor. In such a conventional motor, harmonic noise comprises the 12th order harmonic noise and its integral multiple level harmonic noise, such as 24th order harmonic noise, 36th order harmonic noise, etc.
It's desired to eliminate or at least reduce harmonic noise to make people feel more comfortable, especially in HVAC (heating, ventilation and air-conditioning) applications. One known way to reduce harmonic noise is to provide the rotor poles with grooves running axially along the pole face. These grooves, known as dummy slots as they have no windings laid therein, allow the motor to function as though it has a greater number of poles in terms of harmonic noise but still being easy to manufacture.
However, the air gap between rotor and the stator is increased due to the dummy slots, and motor performance is decreased. Therefore, there is a desire for an improved motor having a reduced harmonic noise.